The present invention relates generally to a system for determining formation characteristics during the drilling of a well. More particularly, the present invention relates to a system for increasing the accuracy of measurements made using nuclear radiation sensors, including particularly neutron porosity and gamma density measurements. Still more particularly, the present invention relates to an improved secondary measurement system for determining tool displacement or "standoff" from the borehole wall, to precisely weight the nuclear radiation measurements based upon standoff distances, thereby improving the accuracy of those measurements.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as "logging," can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or "sonde" is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or "wireline." The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
The sensors used in a wireline sonde usually include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors. See generally J. Lab, A Practical Introduction to Borehole Geophysics (Society of Exploration Geophysicists 1986); D. R. Skinner, Introduction to Petroleum Production, Volume 1, at 54-63 (Gulf Publishing Co. 1981). Nuclear sensors commonly are used to measure neutron porosity and gamma-gamma density of the formation.
The neutron porosity sensor includes a chemical source, which scatters neutrons into the formation. Hydrogen nuclei in the formation slow the neutrons down, and various chemical elements capture the slowed neutrons. When a neutron is captured, a gamma ray is emitted. Slowed neutrons and/or capture gamma rays generally are detected at a near receiver and a far receiver (with respect to the source), thereby indicating the presence of hydrogen in the formation under evaluation. In most instances, the only hydrogen in a formation is that found in water and hydrocarbons. The presence of water and/or hydrocarbons indicates that the formation is porous.
In similar fashion, a gamma ray density sensor includes a chemical source that generates gamma radiation that is focused into the formation. Gamma rays or photons emitted from the source enter the formation to be studied, and interact with the atomic electrons of the material of the formation by photoelectric absorption, by Compton scattering, or by pair production. In photoelectric absorption and pair production phenomena, the particular photons involved in the interaction are removed from the gamma ray beam. In the Compton scattering process, the involved photon loses some of its energy while changing its original direction of travel, the loss being a function of the scattering angle. Some of the photons emitted from the source into the sample are accordingly scattered toward the near receiver and far receiver. Many of the photons never reach the receivers, because their direction is changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process or the pair production process. The scattered photons that reach the receivers and interact with it are counted by the electronic equipment associated with the receiver. The number of gamma rays measured and the energy level of the gamma rays is a function of the density of electrons in the formation, which is approximately proportional to the density of the formation. Examples of prior art wireline density devices may be found in U.S. Pat. Nos. 3,202,822, 3,321,625, 3,846,631, 3,858,037, 3,864,569, and 4,628,202.
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drill string must first be removed or tripped from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers must make some decisions (such as the direction to drill, etc.) without sufficient information, or else incur the cost of tripping the drill string to run a logging tool to gather more information relating to conditions downhole.
More recently, there has been an increasing emphasis on the collection of data during the drilling process. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections, as necessary, to optimize performance. Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
The measurement of formation properties during drilling of the well by LWD systems increases the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Nuclear measurements, such as neutron porosity and gamma-gamma density, commonly are use to provide formation data that is basic to characterizing formation properties. See "State of the Art in MWD," International MWD Society (Jan. 19, 1993). The accuracy of nuclear measurements, however, is limited in LWD systems due to displacement of the source and receiver from the borehole wall. Because the distance that the nuclear source and detectors are displaced from the borehole wall varies during drilling, the usefulness of nuclear based measurement devices is limited in LWD applications. The distance that the source and detectors are displaced from the borehole wall is commonly referred to as "standoff," and the size of the standoff directly affects the measurements made by the nuclear detectors.
In wireline applications, displacement devices are used which force the sonde against the borehole wall to provide a constant standoff distance that thereby insures the integrity of the results achieved by the nuclear sensors. See U.S. Pat. Nos. 3,023,507 and 4,047,027. In LWD systems, however, forced displacement is not a viable option, although in some instances full gauge stabilizers have been used to increase the accuracy of the nuclear based measurement. Full gauge stabilization systems have included arrangements to permit detectors to be mounted in the fins of stabilizers to minimize standoff distance and to eliminate the effects of drilling fluids on the measurements. See U.S. Pat. Nos. 5,250,806 and 4,879,463. Full gauge stabilization, however, is impractical in many conventional drilling applications, particularly when the bottomhole assembly is steered according to the techniques disclosed in commonly assigned U.S. Pat. No. 4,667,751. Unaccounted for motion of an LWD tool implementing nuclear based measurement systems results in a measurement response which is, at best, an average, with varying amounts of drilling fluid present between the formation and the sensors for each measurement, resulting in measurements with poor sensitivity to the formation properties.
Nuclear measurements employ the statistical analysis of nuclear responses or counts measured by the detectors, as well as the energy level measured by the detector. Because data taken with the LWD tool positioned closer to the borehole wall represents the properties of the formation more accurately than measurements taken further from the borehole wall, knowledge of the distance or standoff of the logging tool from the borehole wall has been used to process the data from the nuclear sensors for relative accuracy. See U.S. Pat. Nos. 5,091,644, 5,175,429. Typically, the logging tool continuously rotates during drilling, causing the standoff sensors to change position as the wellbore is drilled. As a result, the determination of standoff distance must be determined continuously and rapidly.
In order to correlate the standoff distance with nuclear detector data, the prior systems have used a system in which nuclear count and energy data is stored in memory by bin locations. See, e.g., U.S. Pat. No. 5,175,429. The data is sorted into a plurality of memory bins based upon a standoff distance for the sensors that is measured contemporaneously with the measurement of the nuclear count data by the nuclear receivers. The bins are predetermined by preset threshold standoff values, so that nuclear count and energy data is stored in a respective bin defined by the associated standoff distance. Thus, for example, in a four bin system, bin 1 could be used to store all count data where a standoff was found between the borehole wall and the sensor of 0-0.25 inches; bin 2 could be used to store all data where a standoff was measured between 0.25-0.50 inches; bin 3 could be used to store all data for a standoff of 0.50-1.00 inches; and bin 4 would be used to store data for a standoff greater than 1.00 inch. As shown in FIG. 6, the segregation of data in this manner permits the weighting of the data when it is ultimately retrieved at the surface of the well (normally when the drill string is pulled to replace or change drilling components). When the data is retrieved at the surface, the data in each bin is assigned a particular weight factor, so that data compiled while the sensor is closest to the borehole wall is assigned the greatest weight, and data taken when the sensor is farthest from the borehole wall is assigned the least weight. The weight factor then is multiplied by each of the count values in that bin. Thus, as shown for example in the prior art technique of FIG. 6, bin 1 is assigned a weight factor of 1 (which is multiplied by all count data values in bin 1), bin 2 is given a weight factor of 0.50 (which is multiplied by all count data values in bin 2), bin 3 is given a count value of 0.25 (which is multiplied by all count data values in bin 3), and bin 4 is assigned a count value of 0.10 (which is multiplied by all count data values in bin 4).
The use of the memory binning technique has enabled the driller to process the nuclear measurements in a more meaningful fashion once the tool is retrieved from the borehole. The binning technique is not, however, without limitations. The first limitation is that additional memory must be included in the system to provide adequate storage space in each of the plurality of bins, in the event that a majority of the data is stored in that particular bin. Consequently, in a four bin system, for example, the downhole memory required may be four times larger than that required if no binning is used. Secondly, the use of memory bins provides a fairly insensitive weighting factor to the nuclear data, which only can be improved by adding more memory bins. In the example given above, the same weight factor would be applied to all count data whether the logging tool is abutting the borehole wall or is 0.25 inches away, because data in that range would be stored in the same memory bin (bin #1). Moreover, in the above example with four bins, if the standoff distance was 0.24 inches, a weight factor of 1 would be used for the associated count data, while measurements occurring with a standoff distance of 0.26 inches would only be assigned a weight factor of 0.50. Such an anomaly in the weight factor function only serves to distort the accuracy of the count data. Moreover, unless a large number of memory bins were provided, which is not practical, the weight factor function will not exhibit a smooth response curve.
In addition, the nuclear data is relatively meaningless until it is appropriately weighted by the standoff distance, which typically does not occur until the data is downloaded from the memory bins at the surface. There is no mechanism in the prior art systems for processing the standoff distance and nuclear data downhole to give a weighted value that can be transmitted to the surface through mud pulses, or which can be used downhole by a downhole controller to make decisions on the fly. As a result, the driller must wait for data to be retrieved and processed at the surface after the data is downloaded from the memory bins.